Recently, hydrofluorocarbon(s) (HFC) having the ozone depletion potential of 0, and lower global-warming potential (GWP) has been used as a refrigerant for a refrigerator and the like. HFC is a stable refrigerant compared to chlorofluorocarbon(s) (CFC) and hydrochlorofluorocarbon(s) (HCFC), and does not largely affect to lubricant oil, an organic material, and a metal. On the other hand, HFC does not have sufficient lubricity, and thus refrigerant oil for HFC is required to have sufficient lubricity. Moreover, the heat generated at a sliding portion accelerates thermal and/or oxidative degradation of refrigerant oil. Therefore, refrigerant oil having high thermal and chemical stability is desired (NPL 1).
And a refrigerant oil is typically circulated together with a refrigerant through part of the refrigeration cycle, and thus the refrigerant oil is exposed to a high temperature range and a low temperature range. In the low temperature range, particularly, part of the refrigerant oil discharged from a compressor may be retained. If the refrigerant oil is exposed in the low temperature range for a long period, the refrigerant oil is crystallized to thereby reduce a circulating amount of the refrigerant in the refrigeration cycle, leading to a cooling failure. Accordingly, it is extremely important to develop refrigerant oil, which is highly stable without precipitating over a long period even in the low temperature range, in view of reliability of a refrigeration device (PTL 1).
Hexaester which is made by reacting dipentaerythritol, with 3,5,5-trimethylhexanoic acid and 2-ethylhexanoic acid is disclosed in PTL 2 as an additive polyol ester of refrigerant oil, and in PTL 3 as a lubricant base oil for compressor refrigeration system. However, the aforementioned hexaester does not satisfy oxidation stability.